1. Field of the Invention
The present invention relates to timing systems, and, more particularly, to methods and apparatuses for measuring phase differences between signals and adjusting interval counters based on the measured phase differences.
2. Description of the Related Art
Timing and digital communication systems routinely use internal clocks to generate reference signals. Those systems use the reference signals to keep time and to generate other signals and codes used to communicate with other devices. In such systems there is a need to know the phases of the internal clocks to synchronize them with other clocks in transmitters and receivers in the communications system.
One timing system that uses frequency standards as timing sources is the Global Positioning System (GPS). The GPS system is a satellite-based spread-spectrum communications system that transmits coded signals from the satellites for use by receivers to determine their position and the time. The GPS satellites use redundant atomic frequency standards (AFS), i.e., atomic clocks, on each satellite as the basis for accurate timing with long-term stability. The atomic frequency standards include Cesium beam frequency standards, or Rubidium based frequency standards. In the GPS satellites the AFS signal is a very accurate signal with a frequency of nearly 13.4 MHz. However, the AFS frequency is determined by the physical attributes of the Cesium or Rubidium atoms, and is not precisely related by any simple ratio of integers to common time-keeping, which is based on the rotation of the earth. Furthermore, the atomic frequency standards are not easily tuned (adjusted).
Each GPS satellite also uses a less stable, but adjustable frequency source, namely, a voltage-controlled crystal oscillator (VCXO) to generate a 10.23 MHz xe2x80x9csystem clockxe2x80x9d which is used to generate timing signals used in the satellite to control the timing of navigation signals broadcast from the satellites. Although the system clock is not sufficiently stable by itself, it is adjustable; and by continually adjusting it using information obtained by comparison with the AFS frequency, the adjusted system clock can obtain the stability of the AFS. By comparing the 10.23 MHz system clock with the very accurate 13.4 MHz AFS clock signal, errors in the system clock can be determined and adjusted. Each GPS satellite uses a phase meter to compare these two clocks and to adjust the system clock. The phase meter data can also be used to monitor the AFS performance, to adjust the satellite timing to follow a world-wide time standard, and/or to create an ensemble clock, that is, to average the timing of multiple atomic clocks from one or more satellites, thus obtaining a virtual clock that is better than any one atomic clock alone.
In many applications, such as in GPS, the phase of a signal and its phase change must be measured with a high degree of precision because of the need to generate the highly stable frequency signals. In some applications where transmitters and receivers are widely distributed and those devices must remain closely synchronized for communications or other purposes, phase meters can be used to help maintain that synchronization. However, in many instances the precision of conventional phase meters is inadequate, thereby inhibiting the development of such systems. In other cases, high precision phase meters are too expensive for certain applications, or the technology used to build conventional high precision phase meters is incompatible with more economical technologies, thereby hindering large scale integration (LSI) of the phase meter.
Conventional methods for detecting a phase difference between two frequency signals, such as the 10.23 MHz GPS system clock and the 13.4 MHz AFS clock, use another clock signal that is very fast with respect to both of the other two frequency signals. Time is measured by counting the cycles of the very fast clock. That fast clock, however, must be as fast as possible, and thus becomes very expensive to achieve even modest precision. A problem with using the fast clock is that the logic technology enabling the clock to operate so fast is expensive making it infeasible to combine that fast logic with more economical logic technology used in large scale integration (LSI). That fast logic technology also consumes more power than does slower, more conventional logic technologies. An effect of the increased power consumption is that the size and weight of ancillary components such as power supplies and drivers must be increased. As a result, the fast logic required by conventional phase meters inhibits the integration of those phase meters with other less expensive logic technology. It also makes it infeasible to include additional phase meters in the satellite for measuring the phase of other signals such as the output of a back-up atomic clock.
Accordingly, there is a need to measure two or more clock frequency signals precisely and economically without requiring use of a faster clock signal in the measurement.
Therefore, in light of the above, and for other reasons that become apparent when the invention is fully described, an object of the present invention is to measure the phase of a signal with a high degree of precision, using signals with lower frequencies than the measured signal.
Another object of the present invention is to integrate a phase meter with other circuits using inexpensive logic technology.
A further object of the present invention is to measure the phase of signals using a phase meter that consumes little power.
Yet a further object of the present invention is to measure a phase difference between two signals without storing a plurality of phase samples.
A still further object of the present invention is to measure a phase difference between two signals using a phase meter with reduced size and weight.
Yet another object of the invention is to facilitate use of multiple phase meters.
The aforesaid objects are achieved individually and in combination, and it is not intended that the present invention be construed as requiring two or more of the objects to be combined unless expressly required by the claims attached hereto.
In accordance with one aspect of the invention, a phase meter measures a phase of two signals by sampling the faster signal with the slower signal, and permuting the phase positions of the samples. For each permuted sample phase position the phase meter determines a phase bin with a phase range encompassing the phase of the sample. For each phase bin the phase meter counts the number of samples with a value of xe2x80x9c1xe2x80x9d that are within the bin""s phase range. Based on the bin counts, the phase meter determines the phase of the first signal according to a formula relating to the bin counts.
In accordance with another aspect of the invention, a phase meter includes sampler that samples a first signal with a second signal, where the first signal has a higher frequency than the second signal. The phase meter also includes a permuter that permutes phase positions of the second signal. A bin assigner is connected to the permuter and sampler, and compares the permuted phase values with a plurality of phase ranges corresponding to phase bins. The bin assigner selects a phase bin for the permuted sample based on the sample phase and the phase ranges of the phase bins. Each phase bin has a bin counter associated with it that counts in response to receiving a signal from the bin assigner indicating that a sample has a phase within the bin""s phase range. A pattern finder determines a phase evaluation formula based on the counts in the bin counters, and a phase evaluator evaluates the phase difference between the first and second signals according to the determined phase evaluation formula and the bin counts, and outputs a phase measurement signal.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following descriptions and descriptive figures of specific embodiments thereof. While these descriptions go into specific details of the invention, it should be understood that variations may and do exist and would be apparent to those skilled in the art based on the descriptions herein.